The invention concerns devices for processing signals in the form of packets and in particular devices of this kind using a processor member having secondary effects likely to generate disturbances affecting adjacent packets.
Devices of this kind are used in time-division multiple access (TDMA) system receivers, for example. The TDMA technique is used in the GSM digital cellular radiotelephone system, for example. In a system of this kind mobile stations communicate by radio with fixed base stations which are connected into the conventional telecommunication network. Communication between a fixed station and mobile stations within its coverage area uses a two-way frequency channel which is time-shared between a plurality of mobile stations, one time slot in a repetitive frame being assigned to each mobile station. The combination of the time slots of these repetitive frames assigned to a given mobile station in this way is sometimes called a channel time slot. In the mobile station to fixed station direction each mobile station sends a data packet in each time slot assigned to it and at its own level. When received at the fixed station the packets from the various mobile stations, which follow on in sequence in the time slots of each frame, do not all have the same amplitude. Because of varying propagation conditions (different distances, obstacles, etc.) their amplitudes are distributed over a broad dynamic range. These differences in level must be compensated at the input of the fixed station receiver.
A solution proposed by the applicant in French patent application n.degree. 92.03754 filed the same day as this application under the title "Dispositif de commande de gain dans un recepteur de signaux d'information" ("Gain control device in a data signal receiver") uses variable gain amplifier means, evaluator means to receive the input signal and to derive from it an amplitude signal, processor means coupled to the evaluator means, receiving the amplitude signal and deriving from it a gain control signal which is passed to a gain control input of the amplifier means, and a delay line between the signal input and the amplifier means supplying to the latter a delayed input signal such that the gain control signal is available to control the gain in the amplifier means when the latter receive the delayed input signal.
The gain control device in this solution further comprises a filter, for example a band-pass filter, to filter the input signal.
The delay line and the filter just mentioned are two examples of processor members having secondary effects likely to generate disturbances affecting adjacent packets.
There is some unwanted coupling between the input and the output of a delay line. If the delay line conveys consecutive packets, as in the system described, the start of one packet may disturb the end of the previous packet.
A passive analog delay line has another unwanted effect, namely the multiple routing of a signal by reflection of the signal at the output, backwards propagation through the line, reflection at the input and further forward propagation to the output which thus supplies not only the input signal delayed once by the line but also an attenuated replica delayed three times by the line. This may disturb the start of the next packet.
A narrowband analog filter such as a quartz crystal filter is itself subject to a ringing phenomenon which extends the length of a packet and may disturb the start of the next packet.
These unwanted secondary effects can naturally be reduced to the point where they are acceptable, but this entails costly precautions.
Also, the situation just referred to of reception of signals in TDMA systems like the GSM system is merely one example and the man skilled in the art will be aware of many other situations of processing signals in the form of consecutive packets where the same problem arises.